This invention relates to methods and apparatuses for testing the authenticity of printed objects, in particular printed sheet material, by measuring light emanating from, in particular reflected or transmitted by, an object to be checked.
To increase forgery-proofness, objects, in particular bank notes, security documents, identification documents or documents of value, are printed in certain surface areas with suitable security inks that convey a certain color effect in the visible spectral region, i.e. in the wavelength region between about 400 nanometers and about 800 nanometers, and additionally have a reflection or transmission behavior characteristic of the particular security ink in invisible, e.g. ultraviolet or infrared, spectral regions. If a security document is imitated with the aid of a color copier, for example, the visible color effect of a printed surface area can be basically reproduced. However, since customary color particles do not have the spectral behavior in invisible spectral regions characteristic of special security inks, counterfeit security documents can generally be recognized by accordingly measuring their reflection or transmission behavior in invisible spectral regions.
Laid-open print JP 52-11992 describes a method and apparatus for testing the authenticity of bank notes. A bank note is irradiated with light from a broad-band light source. Light reflected or transmitted by a place on the bank note is measured in the visible and infrared spectral regions with two photodetectors of different spectral sensitivity. The output signals of the two photodetectors are amplified in a differential amplifier and evaluated in a following threshold and logic circuit. If the difference between the two output signals is within a predetermined range, the logic circuit delivers a binary signal that confirms authenticity or indicates a forgery. This check can be repeated at a plurality of places on the bank note, the authenticity of the note being confirmed when a corresponding signal is delivered by the logic circuit at all or most places.
This method has the disadvantage that the predetermined range of values must be readjusted in the course of the operating lifetime of the apparatus since the sensitivity or dark current of the two photodetectors generally changes to different extents due to aging effects so that the difference of the signals varies. In addition, this method can deliver false results when testing the authenticity in particular of documents soiled in some places or in the case of noisy measuring signals, since only binary evaluation of the difference of the two output signals and thus a yes/no decision on the authenticity of the document to be checked is effected at each place on the document to be checked.
Measurement with two photodetectors one of which is sensitive in the visible spectral region and the other in the infrared is moreover only suitable for testing printing inks having a steplike reflection or transmission course in the transition area between the visible and infrared spectral regions and a substantially constant course in the infrared spectral region.
In the method disclosed in U.S. Pat. No. 3,491,243 the printed sheet material under test is illuminated with white light and the light reflected or transmitted by individual color areas of the sheet material detected by cells sensitive in the visible spectral region that each consist of a photoconductive element with a certain spectral sensitivity and a color filter disposed therebefore with a certain spectral permeability. The material used for the photoconductive elements is for example cadmium sulfide (CdS), which is sensitive to wavelengths below about 550 nanometers. The size of the area to be measured on the printed sheet material can be defined by a convergent lens mounted on a tubular casing.
By this measuring principle only the color of the sheet material is detected and checked by machine. This has the disadvantage that an imitation document showing the same color effect as a real document upon a visual inspection with the human eye cannot be recognized as a forgery using this measuring principle.
In addition, defining the size of the area to be measured on the sheet material by a lens mounted on the tubular casing is bulky and therefore opposes the requirement of a structure as compact as possible. In particular, a change of geometry involving high adjustment effort is required for every desired change of size of the area to be measured on the sheet material.